Many different devices and methods exist for storing digital data. For example, current, conventional computer hard drives use magnetic read and write heads to store and access information from magnetic regions, called bits, on storage media. Data are physically stored as magnetic grains on stacks of platters. An orientation of the magnetic grain in one direction can represent a “1,” and an opposite orientation can represent a “0.” Data are digitally stored on the magnetic storage medium when a read/write head determines the orientation of the grains as bits.
Magnetic storage devices have been used for many years, and the density of bits stored per surface area of the storage medium has increased over time. However, the continued increase of storage density of magnetic media will soon reach physical limitations such as magnetic grain size resistant to thermal self-erasure, difficulty in setting head-to-disk spacings, and switching-speed limitations. Although the evolution of magnetic disks has progressed rapidly, physical phenomena will slow the process that has, in the past, continually increased storage density. A problem arises from the storage medium, whose grain size cannot have a diameter much lower than ten nanometers without thermal self-erasure. Other problems involve head-to-disk spacings that approach atomic dimensions and switching-speed limitations between the head and medium.
Several alternatives to magnetic storage have been proposed, including photochromic-based devices and memories and probe-based data storage. For example, researchers have demonstrated a bit-oriented 3D optical memory system based on a two-photon process using a photochromic spirobenzopyran. See e.g., Dvomikov et al., Opt. Commun., 119:341 (1995); Parthenopoulos et al., Science, 24:843 (1989); and Parthenopoulos et al., Appl. Phys., 119:341 (1990). In this system, two light beams were used to access a point in a volumetric recording medium to write and read data. Two groups have shown that spiropyrans could be used as media for wavelength-multiplexed memory systems. See Hibino et al., Thin Solid Films, 210/211:562 (1992); and Ando et al., Thin Solid Films, 133:21 (1985). Another group used techniques based on atomic force microscopy (AFM) and near-field optics to thermomechanically write on surfaces. See Mamin et al., IBM J. Res. Develop., 39:681–699 (1995). In addition, they compared nitride-oxide semiconductor structures and near-field optical storage to AFM-based storage as potential methods for high-density data storage. A different group used an array of AFM probes to thermochemically store and read back data in thin PMMA films. See Vettiger et al., IBM J. Res. Develop., 44:323–334 (2000). High data rates are achieved by parallel operation of large two-dimensional AFM arrays.